The nascent labor unions of the 19th century sought to rally workers in order to secure favorable wages and working conditions, but two divergent strategies emerged over how to best organize them. Advocates of craft unionism supported the belief that unions should represent workers of a particular trade, such as electricians or shoemakers, while industrial unionists believed unions should represent workers across entire industries.

The earliest unions were craft unions and could trace their philosophic roots to the guilds of the Middle Ages in Europe. Since craft unions revolved around specific trades, their members were predominantly skilled workers. Thus, craft unions wielded greater bargaining power relative to their unskilled industrial counterparts, and their members enjoyed greater solidarity since they all worked in the same trade.

The American Federation of Labor (AFL)—one of the first national unions—organized workers according to the principles of craft unionism. In 1886, a group of craft unionists disappointed by the two major unions of the day, the Knights of Labor and the Federation of Organized Trades and Labor Unions, formed this new national organization devoted to a craft-unionist ideology and led by Samuel Gompers.

The AFL grew steadily by incorporating smaller trade-focused unions into its ranks throughout the 19th century and beyond. Its strict adherence to craft unionism, however, made it inherently exclusionary: The vast majority of skilled workers were white men, and as a result, the AFL imposed membership restrictions on women, immigrants, and African-American workers.

While the AFL grew into one of America’s dominant unions, criticisms of its craft union-stance quickly emerged. Opponents argued that craft unions’ method of only organizing workers by their trade limited labor’s power since they represented a smaller number of workers against increasingly massive industrial conglomerates like U.S. Steel.

Instead, they favored industrial unionism where all workers in an industry would form “one big union,” saying “The longer the picket line, the shorter the strike.” One of the foremost industrial unions was the International Workers of the World (IWW); formed in 1905, it led successful organizing drives and strikes in the early 20th century, but it withered in the aftermath of World War I.

The 1920s were a difficult time for unions amid a business-friendly political and social climate, but the advent of the Great Depression and President Franklin D. Roosevelt’s New Deal gave unions the right to organize and represent workers. In 1935, following unsuccessful efforts to encourage the AFL to adopt principles of industrial unionism, nine labor leaders under John L. Lewis abandoned the AFL and formed an industrial union called the Congress of Industrial Organizations (CIO). The CIO organized workers across entire industries, such as mining and auto manufacturing, and had a membership of more than 2.8 million within seven years of its founding.

The AFL and CIO represented two competing views of labor organization—craft unionism and industrial unionism, respectively—and were bitter rivals, as each fought to poach workers from the other. Eventually, these unions put their differences aside and merged to form the AFL-CIO, which still exists today: As the largest federation of unions in America, it represents more than 12 million workers. The AFL-CIO merger effectively ended the rift between craft and industrial unionism in the United States.